Missing support for ep.nonzero() and ep.flatnonzero()

[zimmerrol]

I think it would be good to have an eagerpy version of the nonzero() and flatnonzero() functions. I guess these can be imitated with a combination of arange and indexing, but it would be more convenient to have a simple drop-in replacement for these two common functions.

[jonasrauber]

Sure, if we can implement this for all frameworks, we could add it.